Neuronal function and cognition are dependent on normal synaptic responses to activity. Impairments in synaptic plasticity are thought to underlie disorders such as Fragile X Syndrome (FXS) and autism. One protein necessary for the regulation of synaptic plasticity is the Fragile X Mental Retardation Protein (FMRP). FMRP is an RNA-binding protein which acts to inhibit translation of target transcripts (which includes its own Fmr1 mRNA) in the postsynaptic compartment until the conferral of an appropriate metabotropic glutamate receptor (mGluR) signal. The absence of FMRP causes FXS. In FXS (FMR1 KO) model mice, this results in altered long term depression (LTD) following mGluR stimulation (mGluR-LTD). Rapid translation of new proteins is required for appropriate synaptic scaling, and similarly, mGluR-LTD is normally protein synthesis- dependent. However, in FMR1 KO mice, mGluR-LTD is aberrantly enhanced and no longer dependent on new protein synthesis. This proposal aims to understand the specific role of newly synthesized FMRP in synaptic plasticity. To approach this we used a mouse model which has an expanded (CGG)120 repeat in the 5'UTR of FMR1 (CGG KI). This is a model of the neurodegenerative disorder Fragile X-associated Tremor/Ataxia Syndrome (FXTAS), which is allelic with FXS. In FXTAS, the expanded (CGG) repeats are transcribed but make translation of new FMRP inefficient. The CGG repeats trigger neurodegeneration as RNA, however patients suffer from cognitive dysfunction which may at least partially result from altered FMRP translation. Using this model we find that CGG KI mice have enhanced mGluR-LTD similar to FMR1 KO mice, but the CGG KI mGluR-LTD remains protein synthesis-dependent. This finding leads me to hypothesize that newly synthesized synaptic FMRP constrains mGluR-LTD by limiting new translation of target mRNAs that are necessary for mGluR-LTD expression. To investigate this hypothesis I propose two specific aims: 1) Determine how dendritic translation of FMRP and known FMRP mRNA targets is altered in CGG KI mice; 2) Determine what features of mGluR-LTD are restored by introducing the poorly translated Fmr1 mRNA in CGG KI mice to FMR1 KO neurons. Aim 1 will be tested using biochemical isolation and stimulation of hippocampal synaptically enriched fractions and local mGluR stimulation of individual dendrites in cultured neurons followed by immunocytochemistry for FMRP and two known FMRP-regulated proteins, Arc and PSD95. Aim 2 will be tested using molecular approaches combined with electrophysiological recordings of synaptic strength in cultured hippocampal neurons. As mental retardation accompanies complete loss of FMRP expression (FXS) and cognitive impairment is associated with mutations that render FMRP poorly translated (FXTAS), this project has the potential to enhance both our understanding of the synaptic properties controlled by rapid FMRP translation and the development of novel treatments for all Fragile X spectrum disorders.